It is generally preferable to use wireless local area networks (WLAN), such as a Wi-Fi network, for data transmission because the wireless local area networks (WLAN) are typically available at lower cost but with higher throughput than mobile networks. Recently, as portable devices which are able to use the wireless local area networks (WLAN) increase, efforts for improving quality and usability of the wireless local area networks (WLAN) have drawn intense scrutiny.